Ceramic mixture having negative temperature co-efficient, a thermistor containing the ceramic mixture and a process for preparing same

ABSTRACT

A ceramic mixture composition having negative temperature coefficient of resistance (NTC), the said composition comprising about 95 weight % of the tetragonal form of Mn 3 O 4  and about 5 weight % of La 2 O 3 , the said ceramic mixture is mixed with stearic acid and wax, compacted and provided with two electrodes disposed from each other to form a thermistor having resistance on the order of mega ohm at a temperature of 25° C. and the resistance of the thermistor drops by almost 40% with every 20° C. rise in temperature and stabilizes to 250±50 ohms at a temperature range of 330° C.±6%.

[0001] This application claims priority to International PatentApplication Serial No. PCT/IB02/00784, filed Feb. 25, 2003.

FIELD OF THE INVENTION

[0002] The present invention relates to a novel ceramic mixture havingnegative temperature coefficient of resistance and a process forpreparing said ceramic mixture. The present invention also relates to athermistor prepared from the said ceramic mixture that can work at atemperature range of 330° C.±6% and a process for preparing the saidthermistor.

BACKGROUND OF THE INVENTION

[0003] A thermistor is a thermally sensitive resistor whose primaryfunction is to exhibit a change in electric resistance with a change inbody temperature. Unlike a wire wound or metal film resistancetemperature detector (RTD), a thermistor is a ceramic semiconductor. Ithas a metal sheathing (stainless steel or Inconel) and contains one ortwo thermocouple Sensing Wires (Chromel-Alumel-K Type orChromel-Constantan-E type) running parallel to the metal sheathing andinsulated from each other and sheathing by a ceramic insulatingcompound. Depending on the type of material used, a thermistor can haveeither a large positive temperature coefficient of resistance (PTC) or alarge negative temperature coefficient of resistance (NTC).

[0004] Thermal sensors can detect temperature, infra-red source and itssize, moving direction and speed, emissivity and wave length. As such,these can find applications in intruder alarms, fire alarms, laserdetection, thermal recording, etc. (Sensors and Actuators, by MoonhoLee,Mina Yoo, A-96(2002) pp. 97-104). NTC sensors now a days are mostcommonly used in automotive applications (Sensors, Vol IV, by W. Gopel,J. Hesse, J. N. Zemel, Vol. 4, (1990)) and in precise temperaturemonitoring devices for temperature measurements, control andcompensation. These sensors can provide precise temperature informationat critical points. These type of sensors are reliable, stable,re-useable and maintenance free. A number of materials have beenreported.

[0005] Thermistors are polycrystalline mixtures of sintered metallicoxides (NiO, Mn₂O₃, and Co₂O₃) or solid solutions (MgCr₂O₄ in Fe₃O₄)that behave essentially as semiconductors. As a result, they havenegative temperature coefficients of resistance. They have provedsuccessfully in a variety of shapes as small, inexpensive, sensitive,fast response temperature sensors, within the range −100° C. to 300° C.Thermistors for above 300° C. are made of oxides of rare earth elements,which are more refractory than nickel and manganese oxides and possesseshigher activation energy. Thermistors for cryogenic use are mostly madefrom non-stoichiometric iron oxides which exhibit very low activationenergy (Sensors, Vol IV, by W. Gopel, J. Hesse, J. N. Zemel Vol. 4,(1990)).

[0006] NTC thermistors consist of metal oxides, such as oxides ofmanganese, chromium, cobalt, copper, iron, nickel, titanium withdifferent stoichiometric ratios, etc., and with different combinations(Measurements, Instrumentation and Sensors Hand Book, by Meyer Sapoff,p. 32.25). These exhibit a monotonic decrease in electric resistancewith an increase in temperature. A number of papers have also beenpublished with different combinations of La—Sr—Mn—O filmLa_(0.75)Ca_(0.25)MnO₃, La_(2/3)Sr_(1/3)MnO₃ pervoskite, etc., formagnetoresistive sensor (4,5,6,7). However, no studies so far have beenreported with 95% Mn₃O₄ and 5% La₂O₃ mixture as NTC material.

[0007] Another reference may be made to Journal of Applied Physics bySam Jin Kim and Chul Sung Kim, Vol. 91, No.1 (January 2001) pp. 221-224.

[0008] Yet another reference may be made to Physics of Manganites by T.A Kaplan and S. D. Mahanti, 1998, p. 201.

[0009] One more reference may be made to Sensors and Actuators, by L I.Balcells, J. Cifre, A. Calleja, J. Fontcuberala, M. Varela and F.Benilez. 81 (2000) pp. 64-666.

OBJECT OF THE INVENTION

[0010] The main object of the present invention is to provide a ceramicmixture having negative thermal co-efficient (NTC).

[0011] Another object of the present invention is to develop a ceramicmixture having long shelf life and that can easily be used in differentenvironments.

[0012] Still another object of the present invention is to provide aceramic mixture that has sufficient flowability for filling in longtubes having internal diameters up to 2.0 mm.

[0013] Yet another object of the present invention is to provide aceramic mixture that can be compacted into a thick mass so that theproperties do not deviate.

[0014] One more object of the present invention is to provide athermistor comprising the ceramic mixture and capable of sensingtemperature in the range of 300 to 350° C.

[0015] Another object of the present invention is to provide athermistor that has resistance on the order of mega ohms at roomtemperature and nearly 250±50 ohms at 330° C.±6% and there may not bemuch variation in its resistance after 350° C.

[0016] Another object of the present invention is to develop a reliablethermistor which has negative thermal coefficient and gives reproducibleresults.

[0017] A further object of the present invention is to provide athermistor for use in different strategic devices like air crafts,armored tanks, explosive store houses etc.

SUMMARY OF THE INVENTION

[0018] Accordingly, the present invention provides a novel ceramicmixture composition containing 95% tetragonal form of Mn₃O₄ and 5% La₂O₃having a negative temperature coefficient of resistance and a thermistorfor sensing temperatures in the range of 330±6%, the said thermistorcomprising the ceramic mixture along with stearic acid and wax forming abase, the said base being provided with first and second electrodes thatare being disposed away from each other. The present invention alsoprovides a process for preparing the ceramic mixture and the thermistor.

DETAILED DESCRIPTION OF THE INVENTION

[0019] The present invention provides-a ceramic mixture compositionhaving a negative temperature co-efficient (NTC) of resistance whencompacted, said ceramic mixture comprising about 95 weight % tetragonalform of Mn₃O₄ and about 5 weight % La₂O₃.

[0020] In an embodiment of the present invention, the ceramic mixturecomposition has resistance on the order of mega-ohms at 25° C. and theresistance value drops to a value between 200 to 300 ohms at 300° C. to350° C.

[0021] In another embodiment of the present invention, the ceramicmixture composition shows increase in potential from −50 mV at 35° C. to13.9 mV at 330° C.

[0022] In yet another embodiment of the present invention, the ceramicmixture composition does not degrade with time.

[0023] In still another embodiment of the present invention, the ceramicmixture composition works at low temperatures as well as hightemperatures.

[0024] The present invention also provides a process for preparing aceramic mixture having negative temperature coefficient of resistance,the process comprising the steps of:

[0025] (a) heating MnO₂ to obtain the tetragonal form of Mn₃O₄;

[0026] (b) cooling the Mn₃O₄ of step (a);

[0027] (c) grinding the Mn₃O₄ of step (b) to obtain Mn₃O₄ of particlesize less than about 60 microns;

[0028] (d) mixing the ground Mn₃O₄ of step (c) with about 5 weight % ofLa₂O₃; and

[0029] (e) grinding and sieving the mixture of Mn₃O₄ and La₂O₃ to obtainthe ceramic mixture.

[0030] In an embodiment of the present invention wherein in step (a),the MnO₂ used is of analytical reagent grade.

[0031] In another embodiment of the present invention wherein in step(a), the MnO₂ is heated up to 1050° C. for a time period ranging between4 hours to 5 hours.

[0032] In yet another embodiment of the present invention wherein instep (b), the Mn₃O₄ is furnace cooled.

[0033] In still another embodiment of the present invention wherein instep (c), the Mn₃O₄ is ground in a mortar and pestle.

[0034] In one more embodiment of the present invention wherein in step(c), the Mn₃O₄ is sieved through a 250 size BSS mesh.

[0035] In one another embodiment of the present invention wherein instep (e), the mixture of Mn₃O₄ and La₂O₃ is ground in mortar and pestle.

[0036] In a further embodiment of the present invention wherein in step(e), the ground mixture is sieved through a 250 size BSS mesh.

[0037] The present invention further provides a thermistor for sensingtemperature, the said thermistor comprising the ceramic mixture havingnegative thermal coefficient along with stearic acid and wax as a base,the said base being provided with first and second electrodes that aredisposed away from each other.

[0038] In an embodiment of the present invention, the ceramic mixturecomprises about 95 weight % tetragonal form of Mn₃O₄ and about 5 weight% La₂O₃.

[0039] In another embodiment of the present invention, the weight % ofstearic acid used is about 1.0.

[0040] In yet another embodiment of the present invention, the weight %of wax used is about 1.0.

[0041] In still another embodiment of the present invention, thethermistor is used for sensing temperature in the range of 300 to 350°C.

[0042] In one more embodiment of the present invention, the resistanceof the sensor drops by 30 to 50% of its original value with every 20° C.rise in temperature.

[0043] In one another embodiment of the present invention, theresistance of the sensor drops by 40% of its original value with every20° C. rise in temperature.

[0044] In a further embodiment of the present invention, the first andsecond electrodes are provided on the surface of the element assembly.

[0045] In an embodiment of the present invention, the first and secondelectrodes are provided inside the element assembly.

[0046] In another embodiment of the present invention, the first andsecond electrodes are made of conducting material.

[0047] The present invention provides a process for preparing thethermistor having negative temperature coefficient of resistance forsensing temperature, said process comprising the steps of:

[0048] (a) heating MnO₂ to obtain the tetragonal form of Mn₃O₄;

[0049] (b) cooling the Mn₃O₄ of step (a);

[0050] (c) grinding the Mn₃O₄ of step (b) to obtain Mn₃O₄ of particlesize less than about 60 microns;

[0051] (d) mixing the ground Mn₃O₄ of step (c) with about 5 weight % ofLa₂O₃;

[0052] (e) grinding and sieving the mixture of step (d) to obtain aceramic mixture;

[0053] (f) adding stearic acid and wax to the ceramic mixture of step(e);

[0054] (g) grinding the mixture of step (f) optionally in the presenceof an alcohol and sieving, and

[0055] (h) compacting and sintering the ground mixture of step (g) andproviding a first and second electrodes to obtain the thermistor.

[0056] In an embodiment of the present invention wherein in step (a),the MnO₂ used is of analytical reagent grade.

[0057] In another embodiment of the present invention wherein in step(a), the MnO₂ is taken in a silica crucible and heated up to 1050° C.for a time period ranging between 4 hours to 5 hours in a mufflefurnace.

[0058] In yet another embodiment of the present invention wherein instep (c), the Mn₃O₄ is ground in a mortar and pastel.

[0059] In still another embodiment of the present invention wherein instep (c), the Mn₃O₄ is sieved through a 250 size BSS mesh.

[0060] In one more embodiment of the present invention wherein in step(e), the mixture of Mn₃O₄ and La₂O₃ is ground in mortar and pestle.

[0061] In one another embodiment of the present invention wherein instep (e) the ground mixture is sieved through a 250 size BSS mesh.

[0062] In a further embodiment of the present invention wherein in step(f), about 1% stearic acid and about 1% wax are added to the ceramicmixture to improve flowability and binding capacity.

[0063] In an embodiment of the present invention wherein in step (g),the mixture of Mn₃O₄, La₂O₃, stearic acid, wax and alcohol is ground ina mortar and pestle.

[0064] In another embodiment of the present invention wherein in step(g), the ground mixture is sieved through a 250 size BSS mesh.

[0065] In yet another embodiment of the present invention wherein ifalcohol is added during grinding in step (g), the sieved mixture isgradually heated to remove the alcohol.

[0066] In still another embodiment of the present invention wherein instep (h), the ground mixture is compacted and sintered to form pellets,and the first and second electrodes are deposited on an outer surface ofthe pellet to obtain the thermistor.

[0067] In one more embodiment of the present invention wherein in step(h), the ground mixture is filled in tubes provided with the first andsecond electrodes, compacted and sintered to obtain the thermistor.

[0068] In one another embodiment of the present invention, the first andsecond electrodes being deposited in step (h) are made of conductingmaterial.

[0069] The present invention relates to the development of a new ceramicmixture which has negative temperature coefficient (NTC) characteristicsfor thermal sensing devices/sensors. The tetragonal form of manganeseoxide (Mn₃O₄) has been mixed with lanthanum oxide (La₂O₃) to form amixture of these compounds. In order to get the desired properties, toincrease its flowability and binding, the mixture was ground (less than60 microns) and mixed with stearic acid (1%) and wax (1%). This materialwhen compacted in the form of pellets or filled in a tube withcompression has a resistance on the order of mega ohms and outputvoltage on the order of −50.00 mV at 30° C. On heating the resistancedrops almost 40% of its value for every 20° C. rise in temperature.

[0070] The MnO₂ used is of A. R. Grade, and was converted to thetetragonal form of Mn₃O₄. The structure was confirmed by XRD technique.Tetragonal Mn₃O₄ was mixed with lanthanum oxide (La₂O₃) in the ratio of95:5. The mixture was ground and sieved through a 250 BSS Mesh size(particle size <60 microns) to obtain the ceramic mixture. For preparingthe thermistor, the ceramic mixture thus obtained is mixed with 1%stearic acid and 1% wax to improve the flowability in the pipe of up to2.0 mm inner diameter. This powder was filled into tubes or compacted toform pellets and sintered at 900° C. for one hour so that materialattains dry strength and has continuity at any two points.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

[0071]FIG. 1 shows a process flow sheet for the preparation of NTCceramic compound working up to 350° C.

[0072] The present invention is described further with respect to theexample, which is given by way of illustration and hence, should not beconsidered to limit the scope of the invention in any manner.

EXAMPLE 1

[0073] Manganese dioxide (MnO₂) Analytical Reagent (AR) quality wastaken in silica crucibles and heated to a temperature of 1050° C. in amuffle furnace. The MnO₂ was kept at this temperature for 4 to 5 hoursin order to convert MnO₂ to Mn₃O₄. The structure of Mn₃O₄ is tetragonaland is confirmed by XRD. The material is then furnace cooled. Mn₃O₄ isthen ground in mortar and pestle and sieved through a 250 size BSS mesh(<60 micron size). This ground oxide is mixed with 5% lanthanum oxide(La₂O₃) and ground thoroughly in mortar and pestle to obtain the ceramicmixture. Stearic acid (1%) and wax (1%) was then added and again groundusing alcohol. It was then slowly heated in order to remove the alcohol.The mixture was again ground and sieved through a 250 BSS Mesh. Thematerial is characterized for potential drop and resistance change. Themixture thus obtained having ceramic powder, steric acid and wax isfilled in about one foot long stainless steel tubes and two sensingwires (chromel and alumel wires) are inserted into the powder inside thetube. The powder is then compacted inside the tube by ramming. Thesetubes were then sintered at 900° C. for 4 to 5 hours so that thematerial becomes a hard mass. The powder can otherwise be compressed atabout 10 tons/inch² in a die to make pellets/tablets. A detailed blockdiagram of depicting the process for preparing the ceramic mixture andthe thermistor thereof is shown in FIG. 1. These ceramic tablets/filledtubes with ceramic powder were characterized for potential drop and dropin resistance. The tests were repeated several times for about one yearto check the stability/repeatability of the test results. The results ofthe test thus conducted are tabulated in Table 1 given below. TABLE 1Composition Manganese Lanthanum Oxide Oxide Resistance Output Voltage S.No. (Mn₃O₄) (La₂O₃) at 30° C. at 330° C. at 30° C. at 330° C. 1 95% 5%3.00 Mega {acute over ( )}Ω 280 {acute over ( )}Ω −50 mV 13.7 mV 2 95%5% 3.20 Mega {acute over ( )}Ω 220 {acute over ( )}Ω −45 mV 13.9 mV 395% 5% 3.37 Mega {acute over ( )}Ω 250 {acute over ( )}Ω −52 mV 13.8 mV4 95% 5% 3.28 Mega {acute over ( )}Ω 260 {acute over ( )}Ω −55 mV 13.5mV 5 95% 5% 4.00 Mega {acute over ( )}Ω 300 {acute over ( )}Ω −50 mV13.9 mV

[0074] Some of the advantages of the present invention are:

[0075] It gives NTC characteristics.

[0076] This material has resistance on the order of mega ohms at 25° C.and this value drops to 200 to 300 ohms between 300° C. to 350° C.Therefore, this material has been used in fire safety sensors/devicesfor strategic applications.

[0077] The material has good flowability. Therefore, it is possible tofill up this material in small diameter and long tubes.

[0078] The material does not degrade with time, temperature and otherenvironmental changes.

[0079] The chemicals are easily available and processable.

1. A ceramic mixture composition having a negative temperatureco-efficient (NTC) of resistance when compacted, said ceramic mixturecomprising about 95 weight % tetragonal form of Mn₃O₄ and about 5 weight% La₂O₃.
 2. A ceramic mixture composition as claimed in claim 1, whereinthe ceramic mixture composition has a resistance on the order ofmega-ohms at 25° C. and the resistance value drops to a value between200 to 300 ohms at 300° C. to 350° C.
 3. A ceramic mixture compositionas claimed in claim 1, wherein the ceramic mixture composition shows anincrease in potential from about −50 mV at 35° C. to about 13.9 mV at330° C.
 4. A ceramic mixture composition as claimed in claim 1, whereinthe ceramic mixture composition does not degrade with time.
 5. A ceramicmixture composition as claimed in claim 1, wherein the ceramic mixturecomposition works at low as well as high temperatures.
 6. A process forpreparing a ceramic mixture of claim 1 having negative temperaturecoefficient of resistance, the said process comprising the steps of: (a)heating MnO₂ to obtain the tetragonal form of Mn₃O₄; (b) cooling theMn₃O₄ of step (a); (c) grinding the Mn₃O₄ of step (b) to obtain Mn₃O₄ ofparticle size less than 60 microns; (d) mixing the ground Mn₃O₄ of step(c) with 5 wt. % of La₂O₃; and (e) grinding and sieving the mixture ofMn₃O₄ and La₂O₃ to obtain the ceramic mixture.
 7. A process as claimedin claim 6 wherein in step (a), the MnO₂ used is of analytical reagentgrade.
 8. A process as claimed in claim 6 wherein in step (a), the MnO₂is heated up to 1050° C. for a time period ranging between 4 hours to 5hours.
 9. A process as claimed in claim 6 wherein in step (b), the Mn₃O₄is furnace cooled.
 10. A process as claimed in claim 6 wherein in step(c), the Mn₃O₄ is ground in a mortar and pestle.
 11. A process asclaimed in claim 6 wherein in step (c), the Mn₃O₄ is sieved through a250 size BSS mesh.
 12. A process as claimed in claim 6 wherein in step(e), the mixture of Mn₃O₄ and La₂O₃ is ground in mortar and pestle. 13.A process as claimed in claim 6 wherein in step (e), the ground mixtureis sieved through a 250 size BSS mesh.
 14. A thermistor for sensingtemperature, the said thermistor comprising the ceramic mixture of claim1 along with stearic acid and wax as a base, the said base beingprovided with a first and a second electrode that are disposed away fromeach other.
 15. A thermistor as claimed in claim 14, wherein the ceramicmixture comprises about 95 weight % tetragonal form of Mn₃O₄ and about 5weight % La₂O₃.
 16. A thermistor as claimed in claim 14, wherein theweight % of stearic acid used is about 1.0.
 17. A thermistor as claimedin claim 14, wherein the weight % of wax used is about 1.0.
 18. Athermistor as claimed in claim 14, wherein the thermistor is used forsensing temperature in the range of 300° to 350° C.
 19. A thermistor asclaimed in claim 14, wherein the resistance of the sensor drops by 30 to50% of its original value with every 20° C. rise in temperature.
 20. Athermistor as claimed in claim 14, wherein the resistance of the sensordrops by 40% of its original value with every 20° C. rise intemperature.
 21. A thermistor as claimed in claim 14, wherein the firstand second electrodes are provided on the surface of the elementassembly.
 22. A thermistor as claimed in claim 14, wherein the first andsecond electrodes are provided inside the element assembly.
 23. Athermistor as claimed in claim 14, wherein the first and the secondelectrodes are made of a conducting material.
 24. A process forpreparing the thermistor of claim 14 having negative temperaturecoefficient of resistance for sensing temperature, said processcomprising the steps of: (a) heating MnO₂ to obtain the tetragonal formof Mn₃O₄; (b) cooling the Mn₃O₄ of step (a); (c) grinding the Mn₃O₄ ofstep (b) to obtain Mn₃O₄ of particle size less than about 60 microns;(d) mixing the ground Mn₃O₄ of step (c) with about 5 weight % of La₂O₃;(e) grinding and sieving the mixture of step (d) to obtain a ceramicmixture; (f) adding stearic acid and wax to the ceramic mixture of step(e); (g) grinding the mixture of step (f) optionally in the presence ofan alcohol and sieving, and (h) compacting and sintering the groundmixture of step (g) and providing a first and a second electrode toobtain the thermistor.
 25. A process as claimed in claim 24 wherein instep (a), the MnO₂ used is of analytical reagent grade.
 26. A process asclaimed in claim 24 wherein in step (a), the MnO₂ is taken in a silicacrucible and heated up to 1050° C. for a time period ranging between 4hours to 5 hours in a muffle furnace.
 27. A process as claimed in claim24 wherein in step (c), the Mn₃O₄ is ground in a mortar and pastel. 28.A process as claimed in claim 24 wherein in step (c), the Mn₃O₄ issieved through a 250 size BSS mesh.
 29. A process as claimed in claim 24wherein in step (e), the mixture of Mn₃O₄ and La₂O₃ is ground in mortarand pestle.
 30. A process as claimed in claim 24 wherein in step (e),the ground mixture is sieved through a 250 size BSS mesh.
 31. A processas claimed in claim 24 wherein in step (f), about 1.0 weight % stearicacid and about 1.0 weight % wax are added to the ceramic mixture toimprove flowability and binding capacity.
 32. A process as claimed inclaim 24 wherein in step (g), the mixture of Mn₃O₄, La₂O₃, stearic acid,wax and alcohol is ground in a mortar and pestle.
 33. A process asclaimed in claim 24 wherein in step (g), the ground mixture is sievedthrough a 250 size BSS mesh.
 34. A process as claimed in claim 24wherein if alcohol is added during grinding in step (g), the sievedmixture is gradually heated to remove the alcohol.
 35. A process asclaimed in claim 24 wherein in step (h), the ground mixture is compactedand sintered to form pellets, and the first and second electrodes aredeposited on an outer surface of the pellet to obtain the thermistor.36. A process as claimed in claim 24 wherein in step (h), the groundmixture is filled in tubes provided with the first and secondelectrodes, compacted and sintered to obtain the thermistor.
 37. Aprocess as claimed in claim 24 wherein the first and second electrodesbeing provided in step (h), are made of a conducting material.